Jun 7, 2017

A Trio of Alien Invaders

By: Lawanda Jungwirth 

         Sometimes landscape plants in our own yards are invasive and we may not even be aware of it. These plants may be the cause of environmental disruption in natural areas nearby or even miles away. Here are three common alien invader landscape plants that are classified RESTRICTED by the Wisconsin DNR. You are not required to remove them from your property but are strongly encouraged to do so. Please look at the lists of native plant substitutes and make the switch!


         Like many invasive plants, amur maple is attractive. The tree has a nice oval shape and the bright green three-lobed leaves are narrower than those of other maples, with serrated edges. They grow to only 20 feet in height and sometimes take the form of multi-stemmed shrubs. In spring, loose clusters of small, pale yellow fragrant flowers cover the tree. In fall, the leaves turn a spectacular scarlet.

                  Amur maples are native to China and Japan and came to North America in the 1860s for use as ornamental plants, as windbreaks for farmsteads and for highway beautification. They were successful on all fronts.

         So what’s the problem? Amur maple produces seeds called samaras (commonly called helicopters) in abundance. Unlike other maple samaras, the “wings” of the amur maple helicopter are almost parallel rather than reaching out in opposite directions. They are lighter than those of other maples and can travel long distances in wind. And they germinate like crazy.

         Along with prolific seed production, amur maples have an additional advantage over native maples in that they are more drought and shade tolerant. They get their start in cultivated landscapes, and from there they invade open grasslands, prairies, forests and field edges. Once there, they out-compete native shrubs, trees and grasses, reduce overall biodiversity and emit chemicals into the soil that inhibit root growth of more desired species.

         To control amur maple, young seedlings are easily pulled from the ground. Prescribed burns are a good method of control for a prairie invasion. Cutting larger trees and painting the stumps with glyphosate is successful. Girdling the tree and painting the cut with triclopyr also works.   (Girdling is the removal of a strip of bark from around the entire circumference of the tree trunk.) Cutting or girdling without pesticide treatment follow-up will result in vigorous resprouting.

         There are some cultivated varieties of amur maple still sold in nurseries that are not restricted in Wisconsin. Cultivated varieties are those with names, like ‘Embers,’ ‘Red Wing,’ ‘or ‘Flame.’ In Minnesota, even the cultivated varieties must come with a warning affixed advising buyers to plant them only in landscapes where seedlings will be controlled by mowing and at least 100 yards from natural areas. Perhaps Wisconsin should follow Minnesota’s lead.


         Russian Olive isn’t an immediately attractive plant, but it does have a rugged sculptural quality when standing alone in a landscape, especially in winter. Like many of today’s worst invasive plants, Russian Olive was intentionally introduced to the United States from Europe in the late 1800s for use as an ornamental landscape plant. It was also planted as a windbreak and on mine spoils since it will thrive on bare mineral soil.

         Today it invades open areas both wet and dry, both sunny and shady. It beats out native species by using water more quickly than they do; in fact it is so efficient at sucking up water that it can dry out riparian areas, thus throwing the whole water’s-edge ecosystem out of whack.

         Russian Olive trees grow to 30’ tall. Leaves are lance-shaped and silvery green. From a distance, the foliage has a gray-green appearance. In spring, fragrant tube-shaped flowers – yellow inside, silver outside - develop in the leaf axils. The fruits are small, hard, dry “olives,” not something humans would eat, but birds love them and freely disperse them to germinate elsewhere. Those seeds not eaten by birds are viable in the soil for up to three years. The bark is thin and easily peels off in long narrow strips.

         Russian Olive is controlled by pulling seedlings as soon as they are noticed. The DNR does not recommend cutting, mowing or burning without follow-up chemical control because plants will just resprout. Here is the DNR’s chemical treatment recommendation: Treat foliage, cut surface, or girdled stem with glyphosate, triclopyr ester, or metsulfuron methyl with a surfactant. Basal bark application of triclopyr ester can also be effective. (Basal bark treatment is appropriate for young, thin-barked trees and involves covering the entire circumference of the tree bark from ground level up to about 1 foot high with pesticide.)



         I had this one in my yard as a privacy screen between our house and the neighbor’s. It is invasive all right – over 24 years I probably spent a few hundred hours pulling baby peashrub seedlings out of the woodchip mulch that surrounded the area. Peashrub is a multi-stemmed tree with compound leaves made up of 8-12 elliptical leaflets. At the end of each leaflet is a small spine. Ouch!

         Pulling the seedlings had to be put on pause each year in early summer when pretty bright yellow flowers covered the trees and thousands of bees attended the flowers.

         After the flowers fade, 1-2” smooth brown pods form. When the pods mature, they eject the seeds as far as they can spit them. ALL of the seeds germinate pretty much immediately.

         The problem with this pretty shrub is that it invades coniferous and hardwood forests, plantations, forest edges, savannas and trails. Coming from Siberia, it is a tough plant, adaptable to many soil conditions and is tolerant of salt and very cold temperatures. Like many invasives, Siberian Peashrub alters soil chemistry to inhibit growth of other plants, including grasses. It also produces chemicals to make itself unappetizing to munching herbivores and to protect itself from many plant diseases, thus insuring its survival.

         As for control, small Siberian Peashrubs can be pulled up or mowed, but mowing must be continual as the plants will resprout. Another non-chemical control is to cover the area with black landscape fabric or heavy duty black plastic in spring just as leaves emerge. Keep the area covered for at least one entire growing season. Repeated prescribed burns may be successful as long as the habitat is appropriate for burning and there is a sufficient fuel load. The DNR’s recommended chemical control includes treating cut stumps with glyphosate or triclopyr, or basal bark treatment in fall with triclopyr.

         Although Siberian Peashrub is RESTRICTED in Wisconsin, three named cultivars are exempt: ‘Lorbergii,’ ‘Pendula,’ and ‘Walkerii.’ Whether exemption was a wise decision or not remains to be seen.